The described light-opaque layer (or light-absorbing layer) is, for example, a black, light-sensitive lacquer. Such light-absorbing layer protects, therein, for example light-sensible areas of circuits with optically integrated elements against incident light, and this layer is, preferably, structured. Examples for black photo resists are carbon-based black resist or pigment-based black resist which both can be used as light-absorbing layer (light shield). This layer is applied at least at the end of the production process onto a passivation layer. It concerns a non-transparent negative photo resist having planarizing properties.
A lacquer having a defined thickness, can also equalize the existing surface topography (small recesses or cavities are filled up with this lacquer). The result will then be a planar surface such that usual alignment marks cannot be recognized anymore by alignment optics because of the substantial covering height of the surfaces of the alignment marks (or of all projections of a respective alignment mark).
Because of the application of a light-opaque layer to be structured at the end of the production process, this layer covers circuits which have previously be inserted into the active areas of the semiconductor-wafer, and can, therein, further on protect the light-sensitive areas there, where no structuring is made, or can make accessible for the light certain light-sensitive areas as optical sensors (optically active elements) in the integrated circuits after the structuring, whereas it can protect other areas against incident light because of the on-going presence of the layer.
Light-opaque, for example shielding layers, so-called black, light-sensitive lacquers or “light-shield-photo resists”, as well as optical protection layers are applied in sensors and circuits of the semiconductor industry, according to their characteristics. The light-layer protects, thereby, for example light-sensitive areas of circuits outside of windows (introduced openings) in a structured, light-absorbing layer. The circuits may respectively be provided with one or several integrated optical element(s). The areas outside of the active optical elements may be protected against incident light. The light-absorbing layer may, however, being generally associated with circuits without optical elements within which light-sensitive areas are to be protected. The protection relates, therein, for example to the suppression of parasitic influences of a light incident onto the circuits. This light protection layer is, for example, produced as originally plain metal-, photo resist, or plastic layer which has reflective and/or absorbing properties depending on the materials used, and which may be structured during the processing sequence.
U.S. Pat. No. 3,969,751 (Drukaroff, RCA) describes a light-absorbing (or light-shielding) layer, for example out of blackened photo resist, blackened metal or plastics which contains carbon particles. The use of a light-absorbing layer in sensors is described, among others in U.S. Pat. No. 4,785,338 (Kinoshita, Canon). For producing this layer, materials like epoxy resin, silicone resin, urethane resin and a kind of black colour or black colouring agent are used.
US 2003/0030055 A1 (Nakano et al.) describes a colour filter containing a light-absorbing layer out of epoxy resin with black pigments. An opaque layer out of light-absorbing, black carbon particles and out of reflective titanium dioxide particles for integrated circuits is described in U.S. Pat. No. 5,031,017 (Pernyeszi, H P).
From US 2004/0032518 A1 (Benjamin, Tower Semiconductor), a light-shielding, structured layer is known with picture sensors wherein this layer contains a light-sensitive lacquer with carbon particles.
Known alignment marks for transparent photo resists consist mostly out of a layer (single layer) for example out of oxide, oxy-nitride or nitride. However, also combinations out of these are perceivable. They are always totally covered by the transparent photo resist. The photo resist protects, thereby, also the alignment marks against etching attacks. FIG. 1 shows the principle of such a projecting alignment mark having a plurality of projections which are arranged in the longitudinal direction of a cavity in the cavity.
In many cases, the structured layers are transparent to light or they are, like the photo resist, transparent at the wavelength of the alignment optics. Thereby, the alignment marks can be optically detected through the photo resist or the structured layer. A general problem with light-absorbing layers is, however, the exact alignment of these layers with respect to prior layers since the photo resists containing carbon or pigments are not transparent in the range of wavelengths of the visible light (350 nm to 800 nm).
In case alignment marks are placed in light-opaque materials, like metal of poly silicone, they cannot be recognized by the alignment optics so that the pinpointed Alignment of these layers to the prior layer is not possible. One manages this with a corresponding surface topography for the application of the at least light-absorbing layers to be structured which is transferred in the shape of a groove in exactly this layer and remains recognizable for the alignment optics.
U.S. Pat. No. 6,153,492 (Wege/Lahnor, Infineon) describes, in this respect, a method for putting up the contrast of alignment marks by depositing and reverse planarizing a tungsten layer. Therein, the trenches around the alignment marks are etched free, for example, back-etched, in an additional step. The depressions formed in this way, are transferred into a subsequently deposited aluminium layer. A modification also stated in the U.S. Pat. No. 6,153,492 includes a back-etch of the oxide layer surrounding the trenches. The tungsten layer now projects beyond the surface of the oxide layer and forms an edge for the exposure of a subsequently deposited aluminium layer.
DE 102 59 322 B4 (Infineon) describes a method for forming an alignment mark in a light-opaque layer on a semiconductor wafer. Therein, trenches having different depths and breadths are formed in the semiconductor substrate that are filled with a first layer. Trenches having a small breadth and depths are completely filled, and trenches having a larger breadth and depth are filled only partly. The latter ones are the trenches for the alignment marks. Thereafter, a second layer is deposited with such a thickness which now also completely fills the trenches of the alignment marks. Therein, the second layer comprises a selectivity in the etching process with respect to the first layer. It follows a chemical-mechanical polishing process in order to achieve a plain surface.
Thereafter, the second layer which is now only present in the trenches with the larger breadth and depth (the trenches of the alignment marks) are etched in an etching process such that these trenches contain new cavities. Since the etching rate of the second layer is by a multiple larger than the one of the first layer, the back-etch depth of other areas of the substrate surface is comparatively small. If now a light-opaque layer is deposited, the new cavities are transferred to the light-opaque layer in case of a sufficient conformity and an adapted thickness of the layer. The position of the cavities can now be recognized by means of alignment optics as alignment mark. This method requires at least one additional layer as well as at least one additional etching step and a polishing process.